But Mikulski was not content to play the role of the token woman. Her time in office has been marked by her passion for putting more women in power. When Mikulski was first elected to the Senate in 1987, she was only one of two women there (the other was Nancy Kassebaum, who was the daughter of a former Kansas governor). Now there are 20, in no small part because of Mikulski’s efforts at mentoring and championing her female colleagues.

Mikulski also experienced, from the inside, the notorious Clarence Thomas/Anita Hill hearings, which cast the Senate’s woman problem in sharp relief. In 1991, these televised hearings showed America an all-white, all-male Senate Judiciary Committee listening, uncomprehending, to Hill’s allegations that Thomas had sexually harassed her. A former aide recalls that Mikulski, with the help of Senator Al Gore and his wife, Tipper, then convened a series of off-the-record, bipartisan consciousness-raising dinners, at which the male senators were treated to gender education from the likes of academic feminists Carol Gilligan and Deborah Tannen, as well as Sam Keen, a California philosopher and proponent of a kinder, gentler masculinity.

This confidence also showed itself when Mikulski helped create an informal women’s caucus in the late ‘90s by bringing the female Senators together for regular dinners. Mikulski’s rule for these dinners: “No staff, no memos, and no leaks.” The idea was that a bunch of bipartisan women gathering in a relaxed setting could come to understand one another and work better together. That hope has paid off, as the women in the Senate are known for being more productive than their male colleagues.

There are 10 times as many women in the Senate than when Mikulski first joined. That number appears to be the tipping point, and now the women don’t all need to hang together based on gender any longer. So Mikulski’s retirement feels very much like the end of an era. When she first joined, female senators were regarded as a novelty. Now, they are still in the minority but are treated with something approaching equality. Mikulski can leave office knowing her place in history has been thoroughly secured.